One example of the above-mentioned pump/motor arrangement resides in the hydraulic drive of vehicle wheels in both on-road and off-road applications. Such arrangements have been the subject of much prior activity and the transmissions employed therewith comprise one or more fixed or variable-displacement hydraulic motors at individual wheels or alternatively motors positioned to drive groups of wheels.
In the most common positive displacement hydraulic machines the fluid chambers undergo cyclical variations in volume following a roughly sinusoidal function. It is known from EP0361927 that a chamber can be left to idle by holding an electromagnetically actuated valve, between the working chamber and the low-pressure source, in the open condition. Thus the output is varied through the action of first filling each working chamber with liquid, then deciding whether to reject the liquid back to the low-pressure source or to pump it at pressure to the output manifold. Pumping the liquid back to the low-pressure source means that a very small amount of power needs to be expended, during the time that a working chamber is idle, whilst still allowing the working chambers to become productive with a minimum latency period.
EP0494236 introduces an additional operating mode which allows the use of the hydraulic machine in a motoring cycle where torque is applied to the rotating shaft, thus allowing a controllable bi-directional energy flow. This type of motor has until now been limited to a control bandwidth and latency of half a shaft revolution, which for example is 17 Hz at 500 RPM, because whole cylinders are selected. However, there are a number of applications where high frequency torque adjustments could offer new and desirable capabilities.
This present invention provides a way of controlling hydraulic motors so as to modulate the output torque at frequencies of up to around 200 Hz and may lend itself to application in a number of fields, some of which are described in detail below by way of background information:
Vehicle Traction Control Systems
An increasingly common requirement in automotive drivelines is that the torque at individual wheels or groups of wheels (for example, rear axle, front axle) must be able to be modulated in both the braking and accelerating modes in order to limit wheel slip. This requirement is due to two factors. Firstly, slipping wheels do not allow the driver to maintain directional control of the vehicle and generally provide less decelerating or accelerating force than wheels that are not slipping. Secondly, the point at which individual wheels or groups of wheels start to slip is different from one another even contemporaneously on an individual vehicle. Slippage is determined by wheel or axle loading, weight transfer during braking and cornering, and the road surface conditions that may be different for different wheels.
Typically in vehicles with a conventional mechanical transmission between engine and wheels the torque modulation is achieved through the momentary application of the friction brakes to one or more wheels, under the control of a central vehicle stability controller. The controller typically takes as its inputs individual wheel speeds, vehicle angular acceleration rates, accelerator pedal position and steering wheel angle, and uses that to modulate the brakes. This system is typically called Antilock Braking System (ABS) when it operates only when braking, or Electronic Stability Control (ESC) when it operates in both braking and accelerating and includes sensors for vehicle movement and acceleration so as to control yaw. As well as applying the friction brakes, the engine torque may be reduced through the use of ignition retarding or interruption, reducing the fuelling rate or adjusting the throttle position.
Other systems exist to vary the distribution of torque to several driveshafts under electronic control. One such system is the E-Diff or Electronic Differential that uses electrohydraulically-actuated friction clutches to distribute torque between two or more driveshafts.
It is essential in all systems that the torque modulation is rapid so as to maintain the optimum slip rate for traction as much as possible. Typical bandwidths are around 10 Hz. ABS brakes, for example, are known to operate at up to 13 Hz. This is not generally high enough to maintain the wheels in the optimal slip condition (generally considered to be about 5-10% slip), but high enough to keep them oscillating between slipping and not slipping.
Electrical Generators
Another application where high torque control bandwidth is desirable is in the driving of electric generators. In this application one or more hydraulic motor(s) may drive one or more synchronous generator(s) for electrical supply to the distribution grid or an isolated power network. The shaft speed of these machines is linked to the AC voltage of the network. The modulation of shaft torque causes a near-instantaneous modulation of the generator current
Harmonic distortion of the current taken by loads on electrical grid, commonly caused by loads such as electronic equipment power supplies, is a high frequency intra-cycle variation from the intended sinusoidal wave, causing a corresponding high frequency intra-cycle variation of required shaft torque. By modulating the torque applied by the hydraulic motor to the generator at high enough frequencies, the current supplied to the grid can be modulated (without requiring complex power electronics) thereby helping to restore the voltage waveform to the required state. This capability also requires accurate control of the phase of the corrections with respect to the generator (and therefore hydraulic motor) shaft.
A separate problem is that the frequency of the AC voltage may start to deviate above or below the desired frequency due to a short term or sudden mismatch between the grid load and grid supply, for example when a new load is turned on or off. In the case that the a load is turned off, the frequency increases above the desired frequency. Reducing the generator torque reduces the power output and restores the correct frequency. If the generator torque can be modulated quickly enough then even very sudden load changes can be accommodated without a change to the grid frequency.
Energy Conversion
A further application where high torque control bandwidth is desirable is when a shaft is driving or being driven from a structure with a large mass that may suffer from vibratory resonances, for example a wind or tidal turbine. If the structure is excited with a torque load having a frequency matching a resonant frequency of the structure, damage to the shaft, attached machines, and the structure may result. By the correct control of the shaft torque at high enough frequency the resonances can be avoided, or eliminated if they have already begun.
From the above it will be appreciated that there are numerous applications in which rapid changes in output torque or power delivery are essential control requirements and many of these fail to employ hydraulic pump/motor arrangements due to their inability to provide adequate control. It is, therefore, an object of the present invention to provide a method of and apparatus for modulating the fluid output from a hydraulic pump/motor which is able to respond rapidly to changes in demand and which may also allow hydraulic pump/motors to be employed in control applications that they have hitherto been excluded from.
According to the present invention there is provided a hydraulic pump/motor with a plurality of cylinders each with a low pressure and high pressure actuated poppet valve under the control of a controller, where the controller is able to provide a sequence of signals to the valves in a phased relationship with the pump/motor shaft so as to effect either a pumping or motoring cycle but has the added capability to modify individual high pressure valve signals in the sequence to lengthen, shorten or adjust the time that the valves remain open, and so to provide for modulation of the pump/motor's torque output.
According to one aspect, the present invention provides: a method of controlling a fluid working machine having: one or more working chambers of cyclically varying volume; one or more inlet valves; one or more outlet valves; a rotating shaft driven by or driving a load; and a controller for receiving data on a first changeable parameter and controlling the operating and closing sequence of said valves to selectively enable said working chamber separately on each of the expansion and contraction strokes of said chamber, so as to supply or accept fluid in accordance with said first changeable parameter, the method comprising the steps of: monitoring a second changeable parameter requiring control at a higher frequency or having a lower latency than said first changeable parameter; and modifying the valve actuation to supply or accept fluid demand in accordance with a combination of said first and said second changeable parameter.
According to a further aspect of the present invention there is provided a fluid working machine having: one or more working chambers of cyclically varying volume; one or more inlet valves; one or more outlet valves; a rotating shaft driven by or driving a load; a controller, for receiving data on a first changeable parameter and controlling the opening and closing sequence of said valves able to selectively enable said individual working chambers separately on each of the expansion and contraction strokes of said chambers, so as to supply or accept fluid in accordance with said first changeable parameter, a monitor, for monitoring a second changeable parameter requiring control at a higher frequency or having a lower latency than said first changeable parameter; wherein said controller monitors said second changeable parameter and modifies the valve actuation to supply or accept fluid demand in accordance with a combination of said first and said second changeable parameter.